net: igmp: Update function stubs when module is disabled
net: igmp: Update function stubs when module is disabled

Current solution throws an error during build if IGMP module is
disabled (statement with no effect).

Update the stubs in a similar fashion as other network modules implement
them.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr